The present invention relates generally to the field of the electromagnetic propagation devices and, more particularly, to electromagnetic transmission line structures and still more particularly to electromagnetic transmission line filter elements.
Spur transmission line filter elements are well known in the prior art and are produced by connecting two adjacent ends of a pair of coupled lines and leaving one line open circuited at the other end. The schematic electric circuit diagram illustrated in FIG. 1 shows the electric equivalent circuit of a spur transmission line. Referring to FIG. 1, first and second coupled lines 12 and 14 are connected electrically at their left ends 16 and the coupled line 14 is left open circuited at its end 18. The length, 1, of the coupled line 14 may be determined, for example, by using a Smith chart such that its end 16 appears short circuited at a specified frequency.
Referring to FIG. 2, a typical realization, in a planar medium such as strip line, of a spur transmission line filter element is illustrated. The spur transmission line filter element 20, as is well known in the prior art, is formed by creating an elbow or spur 22 extending from the strip metallic conductor 24 as is illustrated in FIG. 2. It is noted that the FIG. 2 illustration of the spur transmission line element shows only the metallic conductor portion of the strip line structure and that none of any of the dielectric material that may surround the strip line structure or any waveguide structure containing the strip line conductor element is illustrated in FIG.2.
The even and odd mode impedances Z.sub.0e Z.sub.00 respectively, of the spur line filter element 20 are determined by the widths W.sub.1 and W.sub.2 and gap, W.sub.g of the structure 20 as well as the dimensions and proximity of the ground planes and surrrounding housing, if any (not shown). For example, as the gap G decreases in size, the odd mode impedance Z.sub.00, decreases and the even mode impedance Z.sub.0e increases. Further, as the widths, W.sub.1 and W.sub.2 decrease, the even and odd mode impedance both increase. Often, filer designs require a very large difference between the even and odd mode impedances thereby requiring the gap G to become vanishingly small. While theoretically the dimension of the gap G can be calculated so as to meet the filter design requirements, the actual implementation of the circuit is not realizable because it may be physically impossible to manufacture the circuit with a gap G as narrow as may be required in order to comply with theoretical calculations. In other words, it may be physically impossible to machine the gap G so as to make it as narrow as may be necessary.